Issue 78

N. Mohan et alii, Frattura ed Integrità Strutturale, 78 (2026) 129-145; DOI: 10.3221/IGF-ESIS.78.08

Potentiodynamic polarization during tribocorrosion The potentiodynamic polarization tests were carried out simultaneously with the sliding wear test and the results are as shown in Tab. 4. From the values of Ecorr it is seen that the boronized samples are more active compared to their bare counterparts and the corrosion current density (Icorr) is also significantly higher. The values of Ecorr are more active and higher Icorr values are observed in comparison with the polarization results carried out without sliding wear. This suggests that there is considerable decline in the corrosion characteristics of the boronized MSS during sliding. The passivation tendencies of the boronized layer in NaCl medium is low and the simultaneous sliding action exposes a fresh area after each pass making the system more complex to passivating and stabilizing dynamically, contribution to decrease in corrosion resistance.

Corrosion rate (mm/year)

Elements

Ecorr, mVSCE

Icorr, µA/cm 2

0.0026

13Cr4Ni MSS

Bare

216 ± 22 682 ± 12 728 ± 18 178 ± 24 620 ± 15 661 ± 19

0.22 ± 0.3 16.7 ± 0.7 27.1 ± 0.5 1.0 ± 0.2 14.3 ± 0.5 26.8 ± 0.4

0.195 0.317

Boronized at 950°C Boronized at 1000°C

0.0117

16Cr5Ni MSS

Bare

0.167 0.313

Boronized at 950°C Boronized at 1000°C

Table 4: Corrosion parameters measured during tribocorrosion.

The values of Ecorr and Icorr resulting from potentiodynamic polarization in static and tribocorrosion are compared. For the bare samples, the values of improve while for boronized samples reduction is observed. For bare samples, in the static corrosion tests the kinetics of the oxidation process are dependent on the chemistry between the sample surface and that of the electrolyte solution. In tribocorrosion tests, the decrease in Ecorr values is indicative of good thermodynamic stability in the system and the decrease in Icorr values suggests less material degradation in the given electrochemical system. In the absence of mechanical interruption, the oxide film stays stable. However, during tribocorrosion the rubbing of the counterbody against the surface of the sample causes friction at the surface. This causes a localized increase in temperature which facilitates rapid re-oxidation of the worn surface. Also, the debris particles arising from constant sliding create a pseudo tribo-oxide layer. Due to the rapid development of the oxide layer, the system is more thermodynamically stable. Further, in the case of boronized samples the process of tempering post boronizing has led to partial sensitization of the material. This is confirmed experimentally in one of the previous works of the authors. The depletion of Chromium due to sensitization coupled with boronizing have depleted their presence at the grain boundaries. The pores in the boronized layer facilitate the material degradation due to galvanic coupling. This being the case boronized samples exhibit poor corrosion resistance in static corrosion tests. The condition is further worsened during tribocorrosion as fresh worn surface creates more micro-galvanic points that facilitate increased material degradation phenomenon. Tribocorrosion tests The evolution of coefficient of friction in tribocorrosion conditions is as shown in fig. 10. The values of COF lie in the range of 0.35 - 0.45 for all samples. Highest COF values can be observed for bare 13Cr4Ni and 16Cr5Ni MSS while the lowest COF values are seen in samples boronized at 950°C. The values of COF vary in the range of 0.38 - 0.46 and 0.38 - 0.43 for bare MSS and 0.32 - 0.38, 0.33 - 0.49, 0.32 - 0.48 and 0.28 - 0.36 for the 13Cr4Ni and 16Cr5Ni MSS boronized at 950 and 1000°C respectively. The COF of the bare samples increase steeply upto 5 m, varies moderately between 5 - 18 m and stabilizes beyond 18 m. However, in the case of 13Cr4Ni MSS a slight increase after 35 m and in 16Cr5Ni MSS further drop in the COF values is observed after 38 m which again increases slightly beyond 75 m of the slid distance. Similarly, the boronized samples also demonstrate specific tendencies in the evolution of COF; considering samples boronized at 1000°C, for for 13Cr4Ni and 16Cr5Ni MSS the COF values increase steadily from 0.23 - 0.48 m and 0.22 - 0.34 m respectively, before stabilizing for a second time. Furthermore, for the 16Cr5Ni MSS boronized at 950°C the COF values decrease steeply at 8m of sliding distance which again increases rapidly before stabilizing. The variation in the steady state regions of the boronized MSS can be attributed to variety of factors including formation of tribo-oxide or passive film, the non-homogeneity of the boride layer and three body wear (due to constant debris particles entrapment and removal). The COF values of all the boronized samples in the tribo-corrosion condition are 16 to 33% lower than in the dry sliding wear condition. This could be attributed to the presence of 3.5 wt. % NaCl solution which acts as a

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